The PGSQL_EFFECTIVE_CACHE_SIZE variable specifies the size of the OS data cache that is available to PostgreSQL. PostgreSQL can use that data to select the optimal way to execute requests.
The right value for this variable depends in part on the available RAM on the server where your site is running. Set this value at the highest amount of RAM that you expect to be always available to PostgreSQL.
See What are the right PostgreSQL settings for my site? for values recommended by CollabNet.